Motion Tracking. What is the actual scale for "Set Scale"?
The default scale is blender units...(1m per blender unit), but if you are using different units for the scene then the units will be whatever you want.
The purpose of that option is for you to match known measurements. If you are tracking objects or markers and you know the distance between them, you can use that measurement (along with the lens and sensor size) to make the 3D reconstruction more accurate and closer to the original scene you are tracking.
Read also:
Motion track while knowing distance between points
Does scale of camera have an effect on camera tracking a scene?
Camera tracking reference scene to match different shots in a common space